1. Field of the Invention
Embodiments of the invention relate to power conversion devices, and, in particular, power conversion devices that compensate for alternating current power source voltage fluctuations or power interruptions.
2. Related Art
FIG. 6 shows an alternating current step-up chopper circuit using semiconductor switching elements (hereafter called bidirectional switching elements) that compensates for an alternating current power source voltage drop, supplies a constant voltage to a load, and can control the turning on and off of a bidirectional current (see Japanese Patent No. 3,902,030 at FIG. 17).
The alternating current step-up chopper circuit is configured of a first series circuit wherein a first inductor 4 and first bidirectional switching element 6 are connected in series and a second series circuit, connected in parallel to the first bidirectional switching element 6, wherein a second bidirectional switching element 5 and capacitor 3 are connected in series.
By alternately turning the two bidirectional switching elements 5 and 6 on and off in the alternating current step-up chopper circuit, it is possible to maintain a load voltage Vout (the voltage across the capacitor 3) even when a voltage Vin of an alternating current power source 1 drops. The load voltage Vout is determined by the turn-on and turn-off ratios of the two bidirectional switching elements 5 and 6.
For example, when the power source voltage Vin drops to 80% of the rating, the step-up ratio for keeping the load voltage Vout at 100% of the rating is [1.0/0.8]. Therefore, the turn-on ratio of the bidirectional switching element 5 is [0.8], while the turn-on ratio of the bidirectional switching element 6 is [0.2 (=1−0.8)].
FIGS. 7A-7D show examples of configurations of bidirectional switching elements used in an alternating current step-up chopper. In FIG. 7A, two reverse blocking IGBTs, given a breakdown voltage with respect to voltage of reverse polarity equivalent to that with respect to forward polarity, are connected in anti-parallel. FIG. 7B shows circuits given reverse breakdown voltage by a diode being connected in series to a normal IGBT that does not have reverse breakdown voltage, the circuits being further connected in anti-parallel. FIG. 7C shows reverse conducting elements wherein diodes are connected in parallel to IGBTs, further connected in anti-series. FIG. 7D shows the same kind of connection as FIG. 7C, but MOSFETs are used as the switching elements.
Unlike an IGBT, a MOSFET has resistance characteristics such that current and forward voltage drop are proportional, meaning that, theoretically, the forward voltage drop can be brought ever nearer to zero by increasing the number of MOSFETs in parallel. Also, as a MOSFET also conducts in a reverse direction when voltage is applied to its gate, it is possible under certain conditions to reduce the forward voltage drop farther than with a parallel diode. In particular, as MOSFETs using SiC (silicon carbide) have started to be commercialized recently, a considerable reduction in forward voltage drop is expected.
However, the following two problems are known regarding the previously described alternating current step-up chopper circuit.
The first problem is that there is a limit to the amount of voltage compensation for the alternating current power source voltage drop. In a step-up operation, an input current Iin flowing is larger by an amount proportionate to the step-up of a load current Iout. For example, assuming that the power source voltage Vin drops to [⅕] of when it is rated, the input current Iin momentarily becomes five times the rating. Because of this, the semiconductor switching elements used as the bidirectional switching elements need to be able to tolerate five times the amount of current. Also, it is necessary that the inductor does not become saturated even when the previously described current flows. Because of this, the semiconductor switching elements and inductor increase in size as the voltage range to be compensated for widens, and the cost also increases. Because of this, in actual practice the power conversion device is used with 50% to 100% of the power source voltage as the compensation range, while 50% or less is taken to be outside the compensation range.
However, there is no guarantee that the amount of voltage drop when there is a momentary voltage drop is constant, and while it is preferable to increase the compensated voltage range in order to reduce the risk of failure in a load device, it is not possible to supply power to the load when there is a short power interruption such that the power source voltage drops to zero. Also, as a step-down operation is not possible, it is not possible to compensate for a voltage rise such that the power source voltage Vin becomes higher than the load voltage Vout. Furthermore, when the alternating current power source voltage and load voltage are asynchronous, such as when the power source frequency is abnormal, it is not possible to supply power to the load. Consequently, although the configuration of the alternating current step-up chopper circuit is simple, there is a problem with regard to the level of power source quality and reliability required by the load.
The second problem is that surge voltage is generated when the bidirectional switching elements are cut off, and in the worst case, the elements configuring the load and alternating current step-up chopper circuit are destroyed. As factors in surge voltage being generated, there is one caused when the current to the switching elements is interrupted during a normal working operation, and one caused by an operation turning off all the switching elements when protecting the device, with the latter constituting a particular problem. The former, as is commonly known, is such that a high dl/dt (dl is the amount of current change, while dt is time) occurs when turning off the switching elements, and a surge voltage of L×dl/dt (where L is wire inductance) is generated due to the wire inductance around the switching elements.
To give a description of the latter factor, when some accident such as, for example, a load short circuit occurs during a step-up operation, the bidirectional switching elements 5 and 6 have to be stopped in order to ensure safety. However, when simultaneously turning off the bidirectional switching elements 5 and 6 during a conversion operation, there is no longer a path for consuming energy stored in the inductor 4, meaning that surge voltage is generated in the bidirectional switching element 5 or 6. Regarding the wire inductance, a certain amount of improvement is possible by, for example, shortening the wires between the switching elements, or the like, but as the inductor 4 inductance of the latter factor is determined by circuit conditions, it is extremely large with respect to wire inductance (a few tens of a nanohenry to a few hundred nanohenry), and the surge voltage is also high.
The kind of power conversion device shown in, for example, Japanese patent publication no. JP-A-11-178216 is known as a method of solving the first problem. A configuration is shown in FIG. 8. By energy of capacitors 35 and 36 being supplied via a transformer 31 to a load 2 by an inverter 42 when a power source voltage Vin fluctuates within a constant range, voltage equivalent to the amount of fluctuation of the power source voltage Vin is compensated for, thereby keeping a load voltage Vout constant, and the energy of the capacitors 35 and 36 is replenished or returned by an inverter 43. Meanwhile, when the power source voltage Vin drops to a voltage outside the compensation range, the energy of the capacitors 35 and 36 is supplied to the load 2 by the inverter 43.
It is possible with this device to supply a constant voltage to the load over a wide range of fluctuation of the power source voltage Vin but, as the voltage compensating transformer 31 (such as, for example, an insulating transformer with a commercial frequency of 50 to 60 Hz) is necessary, there is a problem in terms of the capacity, weight, and cost of the device. Also, as power equivalent to the amount of voltage compensation passes through two inverters, another problem occurs in that power converter loss is greater than in the case of an alternating current step-up chopper.
The kind of rectifying snubber circuit shown in, for example, Japanese patent publication no. JP-A-2007-221844 is known as a method of solving the second problem. A matrix converter device 50 of Japanese patent publication no. JP-A-2007-221844 shown in FIG. 9 is configured of a matrix converter 46, an input filter 47, and a rectifier snubber circuit 48. The rectifier snubber circuit 48 is connected to the input side and output side of the matrix converter 46. The input filter 47 is configured of, for example, a inductor and a capacitor. FIG. 9 shows an example of application to a three-phase matrix converter, but the rectifier snubber circuit 48 also achieves the same advantage in a single-phase or three-phase alternating current step-up chopper.
In the case of turning off all of the switching elements when protecting the device, as previously described, surge voltage is generated by energy stored in the inductance on the power source side (herein, the components of the input filter 47) and the inductance on the load side (herein, the motor 49). The surge voltage generated is rectified by passing through a rectifier circuit 51 or 52, and a voltage rise on the power source side and load side is suppressed, and overvoltage prevented, by a capacitor 53 being charged. Also, when the amount of energy generated by the inductance components of the power source side and load side is large, and the direct current voltage of the capacitor 53 rises above a predetermined value, overvoltage is prevented by the energy being consumed in a discharge circuit 56. The operation is such that overvoltage is detected by a voltage detection circuit 57, and the energy is consumed in a resistor 55 by a semiconductor switching element 54 being turned on.
Thus, in the related art, it is possible to address the above-discussed problems by changing or adding circuits, but such solutions do not solve the heretofore described problems simultaneously, and such solutions also can introduce additional problems. Accordingly, as described above, there is a need in the art for an improved power conversion device.